Conventional techniques such as corona discharge treatment, UV irradiation treatment, vacuum glow discharge treatment and electrodeless discharge treatment have been used for improving the surface characteristics of plastic films and metal sheets. Among these techniques, vacuum glow discharge treatment has been extensively used in improving the receptivity of the surface of polymers to adhesives, coatings, water and dyes (see British Pat. No. 891,469 and U.S. Pat. No. 3,309,221).
Nonetheless, vacuum glow discharge treatment has some drawbacks in that the polymer web thus treated becomes excessively hot because (1) the polymer web is directly exposed to an activated plasma, and (2) the polymer web contacts the polymer web guiding rolls that become hot as a result of exposure to the activated plasma.
As described in Japanese Patent Application (OPI) No. 129262/78 (the symbol OPI as used herein means an unexamined published Japanese patent application), when coloration of the treated article is strictly prohibited in the production of photographic materials, the temperature of the article must be controlled within a certain range in order to provide the photographic materials with the desired receptivity to coatings.
If the polymer web of a non-crystalline polymer such as polystyrene becomes excessively hot, the polymer web greatly shrinks and its planarity is impaired such that it is no longer suitable for practical use. The same problem occurs with the polymer web of a crystalline polyester such as a biaxially oriented polyethylene terephthalate, i.e., it experiences partial thermal relaxation or deformation that impairs its flatness. In addition, low-molecular weight substances (e.g., monomers or oligomers) bleed out on the polymer web surface causing the polymer web to lose its transparency such that it is no longer suitable for practical use.
Furthermore, when non-thermosetting resins such as polystyrene and polyethylene terephthalate are heated, their modulus of elasticity is decreased because of their viscoelastic nature and they undergo plastic deformation due to external forces. Therefore, if polymer webs of these polymers are wound on tension applied will cause varying degrees of wrinkles or other surface defects such as the transfer of thicker portions if there is a variation in the polymer web thickness.
Another problem associated with excessive heating of the polymer web is that the active radicals that form as a result of exposure to the activated plasma become increasingly mobile so as to recombine with each other. This impairs the desired surface characteristics of the polymer web.
Therefore, many problems arise if the polymer web is heated excessively by vacuum glow discharge treatment and by contact with heated guiding rolls or left at elevated temperatures after it is wound up to rolls.